PHILADELPHIA — A dozen years ago, a major federal study shattered the deeply held belief that menopausal hormone therapy was the key to keeping women of a certain age healthy, sexy and in good moods.

By showing the therapy did more harm than good, the study shriveled up lucrative hormone sales and set off a wave of lawsuits.

Flash forward. It looks, as Yogi Berra said, like deja vu all over again — this time for men.

Even as female hormone replacement was collapsing, male hormone replacement was catching on.

This wasn’t because of convincing evidence that boosting testosterone helps men fight the toll of aging. It was mostly driven by the introduction of convenient, rub-on testosterone products starting in 2000 and the selling of those products as an answer to low energy, low libido, low mood — what marketers call “Low T.”

Now that the quintessential male steroid is a $2 billion market — with nearly 3 percent of men over 40 estimated to be taking it in 2011 — there are signs of trouble.

Two recent studies suggest testosterone supplementation raises the risk of heart attack, stroke and death. The Food and Drug Administration has begun an investigation, and law firms have begun filing product-liability suits.

In another echo of the female hormone saga, the federal government has stepped in to do the kind of costly, rigorous, placebo-controlled clinical trial that drug companies have no incentive to do. The $50 million Testosterone Trial is now wrapping up and will present results next year, said the leader, University of Pennsylvania endocrinologist Peter J. Snyder.

But size and duration do matter in research.

The Women’s Health Initiative — with 16,600 women taking estrogen-progestin pills for five years — had what scientists call “statistical power.” That’s why, despite criticism, the results have held sway.

The T Trial — 788 men on hormone or placebo for a year — is designed to detect benefits, but not risks.

“It is nowhere near large enough to determine any important risk,” Snyder said. “Not on prostate cancer, or heart disease.”

A government advisory panel said in 2003 that a more definitive trial would be premature, given testosterone’s scant research record. Still, if pluses emerge without the proper counterbalance of minuses, could that fuel misguided use of testosterone?

“This is the problem the (advisers) wrestled with,” said Mark S. Litwin, a University of California, Los Angeles urologist who was on the panel. “There will never be a randomized controlled trial in every area of medicine where one is needed. Sometimes we have to make do with the evidence we have.”

Testosterone is essential for male growth and masculine characteristics such as beards, deep voices and muscle bulk. For decades, testosterone injections have been vital for men with hypogonadism — little or no testosterone — due to birth defects, chemotherapy, infection or other causes.

But the concept of age-related testosterone deficiency, or Low T, remains controversial.

Unlike estrogen, testosterone ebbs but doesn’t bottom out in middle age. Blood levels fluctuate widely during the day and there is no accepted definition of “low” in older men. Some studies suggest boosting it can increase PSA, the marker of prostate cancer; aggravate sleep apnea; enlarge male breasts; and raise stroke risk by overproducing blood-thickening red blood cells.

Testosterone isn’t readily absorbed in pill form and shots are unpleasant, so approval in 2000 of the first topical form, Solvay Pharmaceutical’s Androgel, was a big deal. (Solvay donated its gel to the T Trial.)

“The introduction of topical gel formulations at the turn of the (21st) century has been a milestone in the evolution of the testosterone replacement therapy market,” said Global Industry Analysts Inc. in 2013.

Sales — more than $2 billion last year, according to IMS Health — are forecast to hit $5 billion by 2018.

The FDA says testosterone should be prescribed to men with low levels and symptoms that may be the result of sexual dysfunction or fatigue.

However, an Endocrine Society task force that issued guidelines in 2006 said the level below which symptoms occur “is not known,” and even the panel couldn’t agree what threshold warranted treatment.

One thing the panel agreed on: A diagnosis should be based on “unequivocally low” readings from tests on two separate mornings.

Guidelines, of course, can be ignored.

A recent analysis of a national insurance database found a quarter of testosterone users had no tests of their hormone levels.

Between 2001 and 2011, that usage tripled, to 2.9 percent of men 40 and older.

A British analysis of Medicare claims over the past decade found not only many testosterone-takers with no tests of their levels, but also 9 percent with tests showing normal or even high ranges.

The authors also found testosterone use climbed steadily in the United States while remaining comparatively flat in Britain. The disparity, they speculated, might reflect the fact that Britain doesn’t allow direct-to-consumer marketing of medicines.

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